This element is primarily used in alloys. For instance, when added to lead, it enhances the metal's strength and durability; when alloyed with stainless steel and copper, it makes them more workable. When alloyed with both cadmium and mercury, it forms an infrared-sensitive semiconductor. In addition, it is used in ceramics, glasses, and blasting caps. Bismuth telluride is useful for thermoelectric devices, and cadmium telluride has potential applications in photovoltaic cells for solar power. When zinc is added to cadmium telluride, the product is extremely well-suited for use in solid-state detectors for X rays and gamma rays.

Contents

Yet, tellurium and its compounds should be considered toxic and need to be handled with care. Exposure to even small amounts of tellurium can generate a garlicky odor in one's breath, sweat, and urine. Additional symptoms of exposure to the element or its compounds (at relatively high concentrations) include headache, dyspnea, weakness, skin rash, a metallic taste in the mouth, and blue-black markings on the fingers, neck, face, and gums. Death may occur from pulmonary edema. A person exposed to tellurium compounds should be given medical attention.

Occurrence and production

In nature, tellurium is sometimes found in its elemental form, but it is more often found as the tellurides of gold and silver, such as the minerals calaverite, krennerite, petzite, and sylvanite. Tellurium compounds are the only chemical compounds of gold found in nature. Yet, unlike gold, tellurium itself is also found combined with other elements, forming metallic salts.

The principal source of tellurium is from anode sludges produced during the electrolytic refining of blister copper. In addition, it is a component of dusts from blast furnace refining of lead. Tellurium is produced mainly in the United States, Canada, Peru, and Japan.

Commercial-grade tellurium, which is not toxic if properly handled, is usually marketed as minus 200-mesh powder, but it is also available as slabs, ingots, sticks, and lumps.

History

Tellurium (from the Latin word tellus, meaning "earth") was discovered in 1782 by the Hungarian Franz-Joseph Müller von Reichenstein (Müller Ferenc) in Transylvania. Another Hungarian scientist, Pál Kitaibel, discovered the element independently in 1789, but he later gave the credit to Müller. It was named in 1798 by Martin Heinrich Klaproth who had isolated it earlier.

The 1960s brought growth in thermoelectric applications for tellurium, as well as its use in free-machining steel, which became the dominant use.

In its pure and crystalline state, tellurium has a silvery-white color and a metallic luster. When the element is precipitated from a solution of tellurous acid (H2TeO3) or telluric acid (H6TeO6), it appears to have an amorphous form. There is, however, some debate whether this form is really amorphous or composed of minute crystals.

Tellurium is brittle and can be easily pulverized. When burned in air, it produces a greenish-blue flame and forms tellurium dioxide. In its molten state, the element is corrosive toward copper, iron, and stainless steel.

Chemically, tellurium is related to sulfur and selenium and forms similar compounds. Yet, while sulfur and selenium are nonmetals, tellurium (as well as polonium) is classified as a metalloid.

Tellurium is a P-type semiconductor. Its conductivity, which is higher in certain directions, increases slightly on exposure to light. It can be doped with various metals, including tin, copper, silver, and gold.

Isotopes

There are 30 known isotopes of tellurium, with atomic masses ranging from 108 to 137. Naturally occurring tellurium consists of eight isotopes (listed in the table on the right), three of which are radioactive. Among all its radioactive isotopes, 128Te has the longest half-life (2.2×1024 years).

Compounds

Tellurium can form a variety of compounds. Some examples are given below.

Bismuth(III) telluride (Bi2Te3): This compound is a semiconductor and an efficient thermoelectric material for devices used in refrigeration or portable power generation. Although generally a low-risk material, it can be fatal if large doses are ingested. One should avoid breathing its dust. Also, its reaction with water may release toxic fumes.

Cadmium telluride (CdTe): This crystalline compound is a useful material for solar cells (photovoltaics). It is used as an infrared optical material for optical windows and lenses. It can be alloyed with mercury to make a versatile infrared detector material (HgCdTe). Alloyed with a small amount of zinc, it makes an excellent solid-state X-ray and gamma ray detector (CdZnTe).

Silver telluride (Ag2Te): It occurs in nature in the form of the minerals hessite and empressite. It is a semiconductor that can be doped to have either n-type or p-type conductivity. On heating, silver is lost from the material.

Telluric acid (H6TeO6 or Te(OH)6): It is a weak acid, forming telluratesalts with strong bases.[1] In addition, it is an oxidizing agent. It can be formed by the oxidation of tellurium or tellurium dioxide with hydrogen peroxide or chromium trioxide.

Tellurium dioxide (TeO2 or paratellurite): This solid oxide is the main product of burning tellurium in air. It is highly insoluble in water and completely soluble in concentrated sulfuric acid. It is amphoteric, which means that it can act as an acid or as a base, depending on the solution it is in. It is used as an acousto-optic material. It is also a conditional glass former, meaning that it will form a glass with small additions of a second compound such as an oxide or halide. TeO2 glasses have high refractive indices, transmit into the mid-infrared region of the electromagnetic spectrum, and have properties useful for optical fiber amplification.

Tellurium hexafluoride (TeF6): It is a colorless, highly toxic gas with a foul smell. It is most commonly prepared by passing fluorine gas over tellurium metal at 150 °C. Below this temperature, a mixture of lower fluorides are formed, including tellurium tetrafluoride and ditellurium decafluoride. The physical properties of tellurium hexafluoride resemble those of the sulfur analog, but unlike the latter, it is not chemically inert. It is hydrolyzed in water to form telluric acid, and it reacts with Te below 200 °C.

Applications

Tellurium is mostly used in alloys with other metals. Consider some examples.

Alloyed with lead, it improves the material's strength and durability, and decreases the corrosive action of sulfuric acid.

When added to stainless steel or copper, it makes these metals more workable.

Bismuth telluride (Bi2Te3) is used in thermoelectric devices for refrigeration or portable power generation.

Cadmium telluride (CdTe) has potential applications in solar panels. Some of the highest efficiencies for solar cell electric power generation have been obtained by using this material. It is used as an infrared optical material for optical windows and lenses.

If cadmium telluride is alloyed with some zinc to form CdZnTe, this material is used in solid-state detectors for X rays and gamma rays.

Precautions

Tellurium and its compounds should be considered toxic and need to be handled with care. A person exposed to as little as 0.01 milligrams (or less) of tellurium per cubic meter of air develops "tellurium breath," which has a garlicky odor. The same smell is also present in sweat and urine. The body metabolizes tellurium in any oxidation state, converting it to dimethyl telluride. This product is volatile and smells like garlic.

Exposure to tellurium or its compounds can also cause headache, dyspnea, weakness, skin rash, and a metallic taste in the mouth. In addition, it can produce bluish-black markings on the fingers, neck, face, and gums. Death may occur from pulmonary edema. People exposed to tellurium compounds should receive medical attention.

References

External links

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